1. Field of the Invention
This invention relates generally to tire monitoring devices of the transponder type, and more particularly to a tire monitoring device with an inductive antenna that can be used for transmission of signals by resonant inductive coupling to an antenna used as a fixed antenna connected to the frame.
2. Discussion of the Background
U.S. Pat. No. 4,737,761 shows how it is possible to collect sufficient energy by an inductive coupling between two coils, and how it is possible to use it efficiently. Sending of energy to the wheel is performed by inductive coupling between an antenna connected to a fixed point of the vehicle, for example, the wheel carrier or a brake caliper, and an antenna mounted on the wheel. These antennas will be designated by "fixed antenna" and "wheel antenna," respectively. Numerous variants are possible for constituting and positioning these antennas. Since the basic principle is the use of the inductive coupling, these antennas each comprise at least one coil.
The coil connected to the frame optionally is used both for sending energy to the wheel and for receiving a coded signal sent by the wheel.
To transmit the energy between two coils, the emitting coil should be able to generate a sufficient alternating induction field. In practice, it has been found that to generate this field, it is necessary to apply high voltages, which are not immediately available, across the terminals of the coil. Therefore, it is necessary to adapt the number of turns of each of the coils to obtain a suitable transformation ratio, to add a capacitor to the emitting coil to obtain a series resonant circuit, or to adapt the inductance of the coil to use the stray capacitances that can be considerable at times. At resonance, the voltage across the terminals of the coil is equal to the supply voltage multiplied by an overvoltage coefficient.
By convention, a unit comprising at least the emitting or receiving coil and the capacitance creating a resonant circuit is called an "antenna." In practice, this capacitance is obtained by adding a capacitor in series with the coil. However, in certain installations on the vehicle, the stray capacitance by itself may be sufficient. The capacitor and coil are very close to one another and are located in the same mechanical protective box, to avoid subjecting long electric lines to high voltages.